Nine people have been injured after a police pursuit in the early hours of this morning in Sydney's south-west, with officers saying a 14-year-old boy was behind the wheel of a car that caused a head-on collision.

Police say highway patrol officers were driving along Glenwari Road at Sadleir around 2:30 am (AEDT) when they noticed a stolen Toyota.

They said when they tried to stop the vehicle it sped off.

Police said they called off the chase when the car crossed to the wrong side of the road.

They claim it then continued on to Lurnea where it was involved in a head-on collision with two other cars, injuring the four occupants of each vehicle.

John Brotherhood from the NSW Ambulance Service said two people were trapped in one of the cars when emergency crews arrived.

"We've transported a total of seven people to Liverpool Hospital in various conditions, from lacerated lips to sore backs, necks, and a query fractured rib," he said.

The driver of the stolen car fled the scene but was picked up after a search involving police dogs.

Police said it was a 14-year-old boy, who suffered a broken ankle in the collision.

He has been charged with dangerous driving and not stopping, driving a conveyance without consent, goods in custody and failing to stop after an accident.